Definition of carboxylate in English:

noun

What is more, the indicator has two proton-binding sites: the oxyanion of the xanthene ring and the carboxylate of the benzene as independent sites, which can also exchange protons among themselves.

Such a low pH value suggests that at higher pH the protonation of a carboxylate, forming a hydrogen bond, would break this interaction leading to a destabilization and a change in the heme pocket crucial to catalysis.